The invention relates to a method and a device for fast automatic adaptation of air/fuel ratio for a fuel-injection engine, i.e. an internal combustion engine of the controlled ignition type fitted with a fuel-injection system, and having an oxygen sensor, commonly referred to as a xcex sensor, which detects the oxygen content of the exhaust gases.
The invention therefore relates to fuel-injection engines, in particular for motor vehicles, and a method of automatically adapting the control characteristics governing the fuel supply, i.e. a system of automatically adapting parameters governing charging of the engine cylinders, and which offers an improvement on the automatic adaptation method known from FR-A-2 708 047. The method of rapid automatic adaptation proposed by the invention may simultaneously also be a method of purging a circuit having a canister associated with the engine.
The invention also relates to an automatic adaptation device for implementing the improved method proposed by the invention and incorporates a computer, which computer at least controls the injection system but is preferably an engine control computer which additionally controls at least the ignition process.
It is common knowledge that, for a given type of engine, an adapted engine control parameter or variable, such as the quantity of fuel injected or the injection duration, given that the fuel flow rate-injection duration characteristic of the injectors is known, and which is referred to as a control variable throughout this description, is a known characteristic function which depends on parameters representative of the charging of each of the engine cylinders, referred to as charging parameters throughout this description, and such as the absolute pressure at the air intake manifold, the flow rate of the air admitted to the engine or alternatively the angle at which a throttle valve opens in a valve body on the air intake pipe to the engine, in combination with the speed or rotation speed of the engine. In particular, it is known that the basic fuel injection duration, from which the injection duration effectively applied to the injectors is obtained, is defined as a function of the absolute pressure in the air intake pipe to the engine by means of a characteristic curve which can be likened, in a steady state and across the greater part of the operating range of the engine, to a straight-line curve with a slope G, known as gain, and an initial abscissa D, referred to as shift, for a given engine speed. The increasing linear relationship between the basic injection duration TinjB and the absolute intake pressure P can therefore be written as follows:
TinjB=(Pxe2x88x92D)xc3x97G,xe2x80x83xe2x80x83(1)
where the intake pressure P represents the torque required from the engine, or load, at a given speed.
A known approach to controlling engine operation at an air/fuel ratio of around 1, corresponding to the stoichiometric mixture, is to determine an air/fuel ratio coefficient KO2 which is used to correct the basic injection duration TinjB. This air/fuel ratio coefficient KO2 is derived from a servoloop monitoring the air/fuel ratio R of the air-fuel mixture from an oxygen sensor positioned in the flow of the engine exhaust gas. In practice, the air/fuel ratio coefficient KO2 is between 0.75 and 1.25 and constitutes a multiplicative correction factor for the basic injection duration TinjB, which is therefore corrected by acting on KO2 to an air/fuel ratio R equal to 1. Acting on KO2 generally consists in applying value transitions to this coefficient on either side of a mean value, generally set at 1, for operating the engine in open loop.
Simultaneously, another known approach is to adapt the coefficients D and G automatically as a means of keeping the air/fuel ratio coefficient KO2 as close to its mean value as possible.
Due in particular to manufacturing tolerances, wear and/or the need to replace engine parts or components, the engines exhibit quite different characteristics from one engine to another. However, in order to ensure that engines continue to operate satisfactorily, there is a constant striving towards simultaneously obtaining an air/fuel ratio signal R and an air/fuel ratio coefficient KO2 equal to 1 whilst automatically compensating for tolerances and drifts in engine characteristics by automatically adapting the coefficients D and G of the straight-line curve representing the operation of each engine.
A method of this type for automatically adapting the air/fuel ratio of an injection engine is known from FR-A-2 708 047 and uses a computer which, on the one hand, is connected at least to sensors monitoring engine operating parameters, from which the computer receives at least one engine speed signal and a signal enabling an engine charging parameter P to be determined, this being the absolute pressure in an air intake pipe to the engine downstream of a throttle member such as a butterfly valve controlling the air supply rate, and to an oxygen sensor in the engine exhaust gas, from which the computer receives an air/fuel ratio signal R, and, on the other hand, computes at least values of at least one control variable, namely injection durations to be transmitted to at least one injector, which are obtained from basic values for the control variable TinjB expressed as increasing linear functions of the charging parameter P and represented by straight-line curves, each defined by two coefficients, these being a shift D of the initial charging parameter and a gain G indicating the slope of the line such that TinjB=(Pxe2x88x92D)xc3x97G, each basic value of the control variable TinjB being corrected to generate a corrected value for said control variable TinjCOR taking account of an air/fuel ratio coefficient KO2, to which value transitions are applied as a function of the air/fuel ratio signal R in the operating zones of the engine in closed loop, and fixed at a mean value in the operating zones of the engine in open loop in order to ensure that engine operation is centred an air/fuel ratio R equal to 1, the shift D and the gain G also being automatically adapted in cycles to ensure that the air/fuel ratio coefficient KO2 remains close to its mean value, by correction of any shift of this coefficient KO2 in taking account of the top and bottom values Ph and Pb of the charging parameter for operating points of the engine in a stabilized state.
The teaching of the above-mentioned document is based in particular, for a stabilized engine and depending on certain previous operating conditions of the engine, on enabling the air/fuel ratio to be automatically adapted by modifying at least the shift D and preferably only the shift, within a first operating range of the engine, at low intake pressure (for low charging parameter values) and by modifying at least the gain G and preferably only the gain within a second operating range of the engine, at high intake pressure (for high charging parameter values), these pressure ranges being set.
The disadvantage of this automatic adaptation system is that it is difficult to operate in practice due to the fact that the frequency at which the high-pressure operating range occurs, in the order of 70 kPa, and hence the opportunity of being able to take real and multiple measurements of engine operating parameters during service, is low within a standard cycle when driving a motor vehicle fitted with this engine in the city.
Furthermore, according to the above-mentioned document, whenever an automatic adaptation phase is initiated, it is allowed to continue for a maximum number of n1 cycles at most within the first operating range and for a maximum number of n2 cycles at most within the second operating range and a new automatic adaptation of shift D or gain G is not permitted until after all the automatic adaptation cycles permissible in gain and shift have been performed. The fact that the engine does not operate often enough at the high-pressure range but all the automatic adaptation cycles at high pressure nevertheless have to be run before reverting to automatic adaptation at low pressure means that the engine is not automatically adapted to gain and shift efficiently. This known method therefore has the disadvantage of being too slow in terms of its automatic adaptation function.
The problem underlying the invention is to remedy the disadvantage outlined above and to propose an improved method of automatic adaptation designed to determine dynamically the operating characteristic or line of the engine in its linear section, allowing the gain G and shift D to be computed simultaneously, these being the coefficients relating to the engine charging line.
Another objective of the invention is to propose an improved method of automatic adaptation which will advantageously enable controlled purging of a canister purging circuit associated with the engine, in a manner also known from FR-A-2 708 047, in which it is prohibited to simultaneously have an automatic adaptation phase and a flow rate of a bleed valve of the purging circuit.
To this end, the method proposed by the invention is characterized in that it comprises steps which, for each new cycle of automatic adaptation of the order n, consist in defining a new characteristic line for the control variable Tinj as a function of the charging parameter P on the basis of new coefficients Dnew and Gnew, computed from the charging parameter and control variable coordinates at two points, one of which is at a top value Ph and the other at a bottom value Pb of the charging parameter, and to which corrected values for the control variable TinjCORh and TinjCORb correspond, by applying the formulas:       Gnew    =                            TinjCORh          -          TinjCORb                          Ph          -          Pb                    ⁢              xe2x80x83            ⁢      and                  Dnew      =              Pb        -                  TinjCORb          Gnew                      ,          xe2x80x83        ⁢    and  
validating a value Pk,n, measured when the engine is in a steady state, as a top value Ph,n or respectively as a bottom value Pb,n for the charging parameter, correlating to it a basic value respectively for the top or bottom control variable, in the order n, TinjBk,n taken from an operating line filtered and stored in the computer during the preceding cycle nxe2x88x921 and defined by the stored coefficients DFil,nxe2x88x921 and GFil,nxe2x88x921, and then correlating it to a corrected value for the control variable TinjCORk,n in order to obtain a first point, and taking as the second point respectively the point having the top or bottom value for the charging parameter from the two points stored in the computer during the preceding cycle nxe2x88x921, and having coordinates Pb,nxe2x88x921, TinjCORb,nxe2x88x921; Ph,nxe2x88x921, TinjCORh,nxe2x88x921, and then adopting as the new filtered operating line, defined by new filtered coefficients DFil,n and GFil,n, an intermediate line between the stored line having coefficients DFil,nxe2x88x921 and GFil,nxe2x88x921 and the new line defined by the newly computed coefficients Dnew and Gnew, and storing the new filtered coefficients GFil,n and DFil,n and substituting them for the preceding filtered coefficients GFil,nxe2x88x921 and DFil,nxe2x88x921 to determine the next operating line for the next automatic adaptation cycle.
Accordingly, each new operating line of the engine is defined by its new filtered coefficients GFil,n and DFil,n computed on the basis of the coordinates (charging parameter and corrected control variable needed to obtain stoichiometric air/fuel ratio) of two operating points captured during stable operating phases of the engine, and one of which, having coordinates (Pb,nxe2x88x921, TinjCORb,nxe2x88x921) or, as is the case, (Ph,nxe2x88x921, TinjCORh,nxe2x88x921), is known and located on the preceding filtered operating line, having coefficients DFil,nxe2x88x921 and GFil,nxe2x88x921, stored during the preceding cycle nxe2x88x921 following the last acquisition process, whilst the other point corresponds to a real value of the charging parameter Pk,n measured and validated at a stabilized speed, and a value of the control variable TinjBk,n taken from said preceding filtered operating line, then replaced by a corrected value TinjCORk,n to take account of the value of KO2 acquired simultaneously, the new filtered operating line, having filtered coefficients DFil,n and GFil.n, being part-way between the filtered operating line of the preceding cycle and the new line computed directly from the coordinates of the two operating points thus defined. The values for the new filtered coefficients GFil,n and DFil,n replace the preceding coefficients GFil,nxe2x88x921 and DFil,nxe2x88x921 in memory and the coordinates for the newly acquired point (Pk,n, TinjCORk,n) are also stored and become the coordinates for one of the two points for the next cycle of measurements.
Consequently, a recentering of the xe2x80x9cfirst orderxe2x80x9d is obtained very rapidly by modifying the adaptation terms, being the shift D and the gain G, because:
it is much easier to fulfil the adaptation conditions by suppressing specific ranges of the charging parameter in order to adapt the shift D and the gain G, and
the gain G and the shift D are computed simultaneously and instantaneously so that the adaptation speed is no longer limited by the convergence constraint which imposed very slow variations in the adaptation terms applied using the method known from the aforementioned document.
Advantageously, when the engine is running at a stabilized speed, the method also consists in validating the measured value of the charging parameter Pk,n as the new top Ph,n or bottom Pb,n value respectively only if Pk,n is respectively above a suppressed adaptation band of a predetermined width and having the point with the bottom value stored during the preceding cycle (Pb,nxe2x88x921) as a lower limit, or below said suppressed adaptation band and having the point with the top value stored during the preceding cycle (Ph,nxe2x88x921) as an upper limit. Consequently, a minimum distance between the two points adopted, which is necessary if the computation is to be accurate, defines the permitted adaptation zones. The condition used to validate the value of the newly acquired charging parameter (Pk,n) is that this value is outside the suppressed adaptation band AP, the width of which is predetermined.
Furthermore, with each new cycle of automatic adaptation of order n, the method additionally consists in making a new suppressed adaptation band contiguous with the value entered for the charging parameter Pk,n and comparing this latter value with the lower limit Pb,nxe2x88x921 of the previous suppressed adaptation band so that if Pk,n is lower than Pb,nxe2x88x921, Pk,n will then become the new lower limit Pb,n and the new upper limit will become: Ph,n=Pk,n+xcex94P, xcex94P being the width of the suppressed adaptation band, and if Pk,n is higher than Pb,nxe2x88x921, Pk,n will then become the new upper limit Ph,n and the new lower limit becomes: Pb,n=Pk,nxe2x88x92xcex94P. Accordingly, during a cycle, depending on whether Pk is found to be above or below the suppressed adaptation band stored in memory, Pk will become the new top point Ph or the new bottom point Pb respectively, which determines the upper or lower limit respectively of the new adaptation band that will be suppressed xcex94P.
Furthermore, in order to limit errors in the shift computation, it is necessary to impose a maximum value on the bottom value of the charging parameter, which means that the method will also consist in validating the measured value of this parameter Pk,n as a new bottom value Pb,n only if, in addition, Pk,n is below or equal to a value threshold of the charging parameter, for example in the order of 50 kPa, in calibration, if this charging parameter is the absolute pressure in the air intake pipe, downstream of the throttle member.
In accordance with this method, the engine speed is deemed to have stabilized if, after a predetermined number of transitions in the air/fuel ratio coefficient KO2 around its mean value and if the engine speed N and the position of said throttle member are substantially constant, the difference between the measured value of the charging parameter Pk,n and a measured and filtered value of this parameter PkFil,n is below a value threshold, in which PkFil,n=PkFil,nxe2x88x921+k(Pk,nxe2x88x92PkFil,nxe2x88x921), and where k is a factor between 0 and 1. This being the case, a cycle of measurements and computations to find the coefficients of the new filtered operating line DFil,n and GFil,n is initiated if the measured and filtered value PkFil,n of the charging parameter falls outside the suppressed adaptation band located in the preceding cycle Nxe2x88x921 and Pk is replaced by PkFil in the aforementioned formulas in order to meet said conditions.
Consequently, in accordance with the invention, the coefficients of the engine operating line are stored in the computer and then constantly updated when the engine is operating, during repetitive measuring cycles initiated whenever the engine enters a phase of stabilized operating speed, at a charging parameter value which is outside the suppressed band. The new coefficients resulting from the current measurements take the place of the preceding ones in the memory of the computer.
In accordance with one advantageous embodiment of the invention, an iterative correction of the coefficients defining the engine characteristic is applied more or less progressively in accordance with a logical filtering algorithm in order to avoid too abrupt variations in the operating parameters as they are updated and so as to move progressively towards a mean characteristic. To this end, the method also consists in defining the new filtered operating line, having coefficients DFil,n and GFil,n, by applying a logical filtering process to the new computed coefficients Dnew and Gnew which consists in taking into account only a fraction of the difference between Dnew and Gnew respectively and the preceding filtered coefficients DFil,nxe2x88x921 and DFil,nxe2x88x921 respectively using an approximation of the first order, on the basis of adaptation correction factors KD and KG, which range between 0 and 1 and may be equal, such that:
DFil,n=DFil,nxe2x88x921+KD(Dnewxe2x88x92DFil,nxe2x88x921)
and
GFil,n=GFil,nxe2x88x921+KG(Gnewxe2x88x92GFil,nxe2x88x921).
The filtering rate applied may comprise several levels depending on the adaptation rate of the engine, on the basis of the values assumed by the air/fuel ratio coefficient KO2 and picked up in particular in each of the top and bottom adaptation ranges, i.e. outside the suppressed adaptation band.
To this end, the method also consists in applying adaptation correction factors KD and KG at several levels, depending on the control rate of the engine translated by the value of the air/fuel ratio coefficient KO2, the level of the factors KD and KG being selected depending on the value of KO2 as ascertained in each of the ranges of the top and bottom values of the charging parameter respectively above and below the corresponding suppressed adaptation band.
In a preferred embodiment, a strong, mean or weak value is chosen respectively for at least one of the factors KD and KG depending on whether the air/fuel ratio coefficient KO2 is measured outside a band of the air/fuel ratio coefficient centred on the mean value of KO2 and of predetermined width, in the two charging parameter ranges above and below said suppressed adaptation band, or measured outside said air/fuel ratio coefficient band in one of said charging parameter ranges above or below said suppressed adaptation band but inside said air/fuel ratio coefficient band in the other of said upper and lower charging parameter ranges and, finally, measured inside said air/fuel ratio coefficient band in the two upper and lower charging parameter ranges.
If the computer is switched off, which in practice is when the engine is switched off, the computer memory specifically saves the last coefficients DFil and GFil stored, which will define the initial operating line of the engine next time the computer is switched on, which in practice is when the engine is next started. A specific initialisation system allows typical coefficients to be loaded whenever the computer is switched back on.
To this end, every time the engine is started, the method also advantageously consists in determining, by means of the filtered operating line, having coefficients DFil and Gfil, stored in memory on re-starting, two theoretical values for the control variable TinjCORh and TinjCORb corresponding to two values of the charging parameter selected from outside the usual range of values for said charging parameter and which are a top initialisation value PhINIT and a bottom initialisation value PbINIT respectively, selecting a suppressed adaptation band essentially centred between PbINIT and PhINIT, with a lower limit Pb higher than PbINIT and an upper limit Ph lower than PhINIT, after which the measuring and computing cycle is then run as in the continuous state, a new validated value being acquired for the charging parameter if said new value falls outside the suppressed adaptation band and the coefficients DFil,n and GFil,n for the new filtered operating line being computed on the basis of the new measured and filtered value of the charging parameter PkFil and one of the two initialisation value points PhINIT or PbINIT of said parameter. During the measuring and computing cycle following a re-start of the engine, the filtered value of the air/fuel ratio coefficient KO2 is also regarded as its mean value, i.e. 1 if KO2 is a multiplicative factor for correcting the basic values to generate corrected values of the control variable. Furthermore, when the engine is re-started, it is of advantage if the computer is adapted progressively to the real conditions by setting the initial values for the adaptation correction factors KD and KG as a function of a fictitious degree of engine adaptation, for example assuming that KO2 is inside said air/fuel ratio factor band for each of the top and bottom bands of the charging parameter which are respectively above and below the suppressed adaptation band.
Furthermore, before the computer is switched on for the first time, the method also consists in pre-loading initial values GINIT and DINIT of the operating line coefficients into the computer memory, which are defined experimentally for the specific type of engine and substituting them for the coefficients GFil and DFil stored for start-up purposes and not yet existing.
If the engine co-operates with a purging circuit, fitted with a canister to collect fuel vapours from at least one tank and connected to the engine intake pipe by an electrically controlled valve for purging the canister, and whose rate is driven by the computer so that the simultaneous timing of the purging valve and of the automatic adaptation system is prohibited, as disclosed in FR-A-2 708 047, it is of advantage to provide additional steps in the method proposed by the invention so as to supplement it, associating the automatic adaptation strategy with a strategy for purging the canister, priority being assigned to one or the other of the two strategies depending on the engine adaptation level and the degree to which the canister is filled. If the canister is very full of fuel vapours, the automatic adaptation will be inhibited. In the reverse situation and if the engine is not being sufficiently adapted in a top or bottom range of the charging parameter, i.e. if KO2 is not within said air/fuel ratio coefficient band in this top or bottom range, adaptation will take priority in this specific range of the charging parameter. The priority between adaptation and purging the canister is managed by modulating the width of the suppressed adaptation band. This suppressed adaptation band is fully reserved for the canister purging, and the wider this suppressed band, the more the canister purging has priority. In accordance with the method, it is therefore sufficient to modulate this width relative to a nominal value of the suppressed adaptation band in order to manage the priority between adaptation and purging the canister.
To this end, the method also consists in widening the suppressed adaptation band respectively towards the top values or towards the bottom values of the charging parameter if the engine regulation rate is satisfactory, depending on the value of the air/fuel ratio coefficient KO2, within the respective top or bottom range of the charging parameter which is above or below said suppressed adaptation band respectively before it is widened.
However, in order to reactivate the adaptation options, the method advantageously also consists in making the widening of the suppressed adaptation band effective only during a predetermined period of time, assisted by a counter which is re-started with each automatic adaptation cycle to count down said period of time. Furthermore, so as to take account of how full the canister is, the method may also consist in defining an estimated coefficient KCAN of the fuel contents in the purging circuit, in the manner described in FR-A-2 708 049, to which reference may be made for further information and advantages on this subject. All that will be said here is that this coefficient KCAN may be worked out when purging is permitted on the basis of the deviation of the air/fuel ratio coefficient KO2 so that KCAN is increased or decreased respectively if KO2 is respectively below or above its mean value. The method therefore consists in entering an automatic adaptation phase if KCAN falls below a predetermined threshold relating to the fuel content.
Another objective of the invention is a device for automatically adapting the air/fuel ratio of an injection engine, comprising a computer connected to sensors detecting operating parameters of the engine as well as an oxygen sensor in the exhaust gas of the engine, said computer computing values of a control variable such as the injection duration applied to at least one fuel injector of the engine, and obtained from base values TinjB expressed as increasing linear functions of a charging parameter of the engine, such as the pressure P in an air intake pipe of the engine, with a shift D from the original charging parameter and a gain G corresponding to the slope of the corresponding characteristic line, said base values TinjB of the control variable being corrected by means of an air/fuel ratio coefficient KO2 determined by the computer as a function of the air/fuel ratio signal R from the oxygen sensor operating in a closed loop and equal to a mean value when operating in an open loop, in order to centre the engine operation on an air/fuel ratio equal to 1, the computer automatically adapting the shift D and the gain G in cycles to ensure that KO2 remains close to its mean value by correcting any deviation in KO2 and which is characterized in that the computer comprises at least one microprocessor programmed and/or set up so as to control running of the method proposed by the invention as described above.
Other advantages and features of the invention will become clear from the following description of an example of an embodiment, which is not restrictive in any respect, and with reference to the drawings, of which: